Proxima Flare May Force Rethinking of Dust Belts

byPaul GilsteronFebruary 27, 2018

News of a major stellar flare from Proxima Centauri is interesting because flares like these are problematic for habitability. Moreover, this one may tell us something about the nature of the planetary system around this star, making us rethink previous evidence for dust belts there.

But back to the habitability question. Can red dwarf stars sustain life in a habitable zone much closer to the primary than in our own Solar System, when they are subject to such violent outbursts? What we learn in a new paper from Meredith MacGregor and Alycia Weinberger (Carnegie Institution for Science) is that the flare at its peak on March 24, 2017 was 10 times brighter than the largest flares our G-class Sun produces at similar wavelengths (1.3 mm).

Image: The brightness of Proxima Centauri as observed by ALMA over the two minutes of the event on March 24, 2017. The massive stellar flare is shown in red, with the smaller earlier flare in orange, and the enhanced emission surrounding the flare that could mimic a disk in blue. At its peak, the flare increased Proxima Centauri’s brightness by 1,000 times. The shaded area represents uncertainty. Credit: Meredith MacGregor.

Lasting less than two minutes, the flare was preceded by a smaller flare, as shown above, revealing the interactions of accelerated electrons with Proxima Centauri’s charged plasma. We already knew that Proxima produced regular X-ray flares (recent studies have pegged the rate at one large event every few days), though these are much smaller than the flare just observed. The effects of such flaring on Proxima b could be profound, according to MacGregor:

“It’s likely that Proxima b was blasted by high energy radiation during this flare. Over the billions of years since Proxima b formed, flares like this one could have evaporated any atmosphere or ocean and sterilized the surface, suggesting that habitability may involve more than just being the right distance from the host star to have liquid water.”

The issue has significance far beyond Proxima because M-class stars are the most common in the galaxy. They’re also given to pre-main sequence periods marked by frequent changes in luminosity, and prone to a high degree of stellar activity throughout their lifetimes. Proxima Centauri, spectral class M5.5V, has long been known to be a flare star, leading to the current interest in determining the effects of its variability on the single known planet.

MacGregor and Weinberger worked with data from the ALMA 12-m array and the Atacama Compact Array (ACA). The datasets from these observations were examined by Guillem Anglada (Instituto de Astrofísica de Andalucía, Granada, Spain) in 2017, whose team found signs of a dust belt of about 1/100th of Earth’s mass in the 1-4 AU range, with the possibility of another outer belt. These two structures seemed to parallel the asteroid and Kuiper belts we find in our own Solar System, and it was thought that the inner belt might help us constrain the inclination of the Proxima Centauri system, while giving us an idea of its complexity.

I should pause to note that Guillem Anglada is not Guillem Anglada-Escudé, who led the work that discovered Proxima b — the similarity in names is striking but a coincidence. Making this even more confusing is the fact that Guillem Anglada-Escudé is a co-author on the dust paper on which Guillem Anglada was lead author. If we can get the names straight, we can go on to note that the MacGregor/Weinerger results question whether the dust belts are really there.

For MacGregor and Weinberger looked at the ALMA data as a function of observing time, noticing the transient nature of the radiation from the star. From the paper:

The quiescent emission detected by the sensitive 12-m array observations lies below the detection threshold of the ACA observations, and the only ACA detection of Proxima Centauri is during a series of small flares followed by a stronger flare of ∼ 1 minute duration. Due to the clear transient nature of this event, we conclude that there is no need to invoke the presence of an inner dust belt at 1 − 4 AU. It is also likely that the slight excess above the expected photosphere observed in the 12-m observations is due to coronal heating from continual smaller flares, as is seen for AU Mic, another active M dwarf that hosts a well-resolved debris disk. If that is the case, then the need to include warm dust emission at ∼ 0.4 AU is removed. Although the detection of a flare does not immediately impact the claim of an outer belt at ∼ 30 AU, the significant number of background sources expected in the image and known high level of background cirrus suggest that caution should be used in over-interpreting this marginal result.

So we still have the possibility of an outer belt, though not one that can be claimed with any degree of certainty, and we have removed the need for the inner belt. Proxima Centauri may indeed have other planets and a more complex system than we currently know, but the data from ALMA and ACA now appear to be indicative only of the known flaring phenomenon.

“There is now no reason to think that there is a substantial amount of dust around Proxima Cen,” says Weinberger. “Nor is there any information yet that indicates the star has a rich planetary system like ours.”

But the flare and its reflection off of Proxima b is best imaged in the UV and JWST is designed for the IR. It also needs to be monitored continuously, so if a flare occurs when not facing toward earth the large scopes could do a quick observation to see the much larger signal to noise ratio from the reflection of of Proxima b.

These two arrays of telescopes will cover the northern and southern sky in the blue end of the spectrum, good for picking up UV flares. They are already monitoring Proxima Centauri and may be able to detect exoplanets magnetic fields. See Pdf file below!

“The Evryscope (“wide-seer”) is an array of telescopes pointed at every part of the accessible sky simultaneously and continuously, together forming a gigapixel-scale telescope monitoring an overlapping 8,000 square degree field every 2 minutes. Funded by NSF/ATI and NSF/CAREER, and operating at CTIO since May 2015, the Evryscope-South will soon be joined by the Evryscope-North to give truly-all-sky coverage.”

If Proxima b has a strong magnetic field there could also be inductive and tidal heating creating a planet similar to Io. This type of environment could lead to ocean planet with oceanic ridge systems churning the surface over in a relatively short time period. The ash and dust from the very active volcanic eruptions would also shield the planets atmosphere and oceans from the evaporating effect of the flares. This takes place on earth in a much slower process, the question is if enough organic chemicals and liquid water could survive, but if Proxima b has cometary showers this would also resupply those requirements for life. The more important question is how this type of planet, with possible a much different organic and mineral composition would be able to hold onto its atmosphere and oceans as it is recycled them thru the oceanic lithosphere.

Oceanic lithosphere.

“Oceanic lithosphere consists mainly of mafic crust and ultramafic mantle (peridotite) and is denser than continental lithosphere, for which the mantle is associated with crust made of felsic rocks. Oceanic lithosphere thickens as it ages and moves away from the mid-ocean ridge.”

I predict that any complex life we find on planets around red dwarfs will be immigrant life forms. There may be some simple life on some planets around red dwarfs (though even that may prove exceedingly rare), but I seriously doubt if any complex evolution takes place on tidally-locked planets orbiting very close in to stars prone to extreme solar flare activity.

But there is a big reason for planets around red dwarfs to be immigration magnets: their long life. Any species that solves the interstellar travel and migration problems is surely not bothered by solar flares. And what a great place to set up a colony, around a star with a possible ten-trillion-year-plus lifespan!

I think we’ll find that any civilizations with interstellar capability (not too mention sophistication, class and even grace) will be highly attracted to planets around red dwarf stars.

I tend to disagree with that, I think rather the opposite: for a truly interstellar civilization, it would be relatively easy to cherry-pick the most suitable real estate, rather than go for a 3rd choice home, just because you an stay there very long.
I mean: a quiet late G or early K star (G8-K0/1) would still give you on the order of 10-30 stable and pleasant gigayears. A planet would then have to orbit at about 0.7 AU (Venus distance) to have roughly earthlike conditions, distant enough to avoid tidal locking.

If you have a camel, you don’t settle in the desert, but you travel from oasis to oasis.

‘I tend to disagree with that, I think rather the opposite: for a truly interstellar civilization, it would be relatively easy to cherry-pick the most suitable real estate, rather than go for a 3rd choice home…’

A truly interstellar species would be at home anywhere around any star and even between the stars.

I think the opposite might take place for interstellar-capable civilizations approaching Kardashev-II level. They’ll go to OB-type stars to get as much power as possible. Even MYr there is quite a long time, life cycle would be “spend some millions of years around the star, then some hundred years migrating to another star in OB-associations, or tens of thousands of years migrating to a new stellar nursery”. Ultimately, they will go to the environments with highest power density on parsecs scale, if they don’t turn off completely from energy-extensive path.

It’s likely that Proxima b was blasted by high energy radiation during this flare. Over the billions of years since Proxima b formed, flares like this one could have evaporated any atmosphere or ocean…

If that means the atmosphere is [mostly] gone, then detecting atmospheric spectra is going to be a problem. The biosignature for even lithophilic life will be absent or very hard to detect, so that we may even have a false negative for life.

I am skeptical of the existence of the notion that a high percentage of red dwarf stars have habitable planets. The issue of flares such as the one mentioned here may be the most daunting piece of evidence against the habitability of these systems. I suspect that most habitable planets exist around G and K stars. That said, red dwarfs are much more common than G and K stars. I guess an important question might be as follows: are the high numbers of red dwarfs enough to offset their downsides in terms of suitability for life such that perhaps there may be approximately equal numbers of habitable planets around M stars as there may be around G and K stars?

It isn’t really unexpected, though – the activity levels are pretty much in line with the trends seen in other red dwarf stars. See West et al. (2015) “An Activity-Rotation Relationship and Kinematic Analysis of Nearby Mid-to-Late-type M Dwarfs” – from the abstract: “we find that all M dwarfs with rotation periods shorter than 26 days (early-type; M1-M4) and 86 days (late-type; M5-M8) are magnetically active.” Proxima is an M6 star with a rotation period is about 83 days, so falls into the stellar class/rotation regime where all the stars are found to be active.

The reason for the divide between the activity cutoff for early and late-type M dwarfs may have something to do with the transition to a fully convective interior in late-type stars, which also occurs around a spectral type of M5.

It’s high metals content will not help either and Proxima is not the worst out there though either ! If a planet around Proxima had an ozone layer it would simply be destroyed in short order with the largest flares.

One silver lining of a tidally locked planet is that the dark side is relatively immune from such flares. Also, recent work has shown the possibility of weather mixing between both sides, thus avoiding excessive cold on the dark side. Thus around the terminator boundary seems a reasonable place to expect life, despite such flares.

Perhaps flares make most red dwarf star systems too hazardous for complex ecologies with life forms using liquid water as their working fluid. But what about close-in planets like subjovians or large moons like Titan with liquid methane/ethane or ammonia lakes or oceans? Those flares might really speed up and increase the production of complex organic compounds.

The solar wind is what causes a planet to loose an atmosphere since like plasma, it has it’s own electric and magnetic field build into it which will accelerate particles in the upper atmosphere away from the planet to escape velocity. This happens on both Mars and Venus since neither have a magnetic field strong enough to deflect the solar wind; Earth has a large magnetosphere which blocks the solar wind. Venus and Mars have don’t have one, so they have already lost a lot of water and atmospheric gases that way including the splitting of molecules through UV radiation.

The solar flares generate x-rays by the acceleration of electrons in their magnetic fields which follow the loop down into the photosphere or surface where they collide with matter and create bremsstrahlung deceleration or braking radiation which creates X-rays. The spectral signature of solar flares is in the extreme ultra violet so we can differentiate between the atmosphere of an exoplanet and solar flare spectra. We can expect loss of atmosphere from Proxima b, but not all of it might be lost. Venus has not lost all of its atmosphere in spite of a continual loss due to the solar wind.

We keep assuming that these red dwarf planets will be 1:1 tidally locked. If there’s any appreciable eccentricity in their orbits, as I understand it many (most?) might be caught in a Mercury-like 3:2 resonance. The effect of this on habitability is anybodies guess – they will still be slowish rotators.

Pardon me, the solar wind has a magnetic field so when it moves past the upper atmosphere of a planet, it generates an electric field like a motor which “accelerates charge gas atoms or ions” out of an atmosphere.
NASA Mission Reveals Speed of Solar Wind Stripping Martian Atmosphere

A one off event of this strength would do little damage to the atmosphere and no direct damage to life. This event was observed in 1.3mm so we don’t know how strong it was in other wave lengths. However, I would guess that if it occurred here it would have major socio economic impact. It would knock out all exposed active satellites and induce currents in overland power lines that would destroy transformers en masse essentially destroying the electrical grid. Existential problems for an atmosphere or ecosystem would arise only after repeated exposures over a very long period. Proxima Centauri produces visual flares over 10 e33 erg in strength around 8 times a year. That is a lot. For comparison, the strongest solar flare(from the sun) by far in recent history occurred 150 years ago and was only around a tenth of this strength.

Although the planet may now be dry due to the conditions to which it has been subjected in the past that may not be the actual case. Proxima is near the limit of been captured by Alpha Centauri, so it could have formed in orbit, been captured later in the denser birth cluster or it may have even been capture much, much later on. A way in which Proxima could have been captured is if the stars past close to each other fairly recently and interacted with AC’s Oort cloud removing momentum and allowing orbit. If that was the case a lot of water could have been delivered into Proximas inner system watering any planets.

Andrew Palfreyman. One can’t have a Titan like world in the life belt of any class star because methane is a gas a room temperature and liquid methane will evaporate or immediately become a gas in the life belt since it’s liquid and freezing temperature is very low so it has to be far from a star in the ice belt. Titan would lose most of its atmosphere in the life belt.

Quote by Ronald: “But as you undoubtedly know, the main issue of Venus is not a lack of (any) atmosphere, but a tremendous lack of H2O, because of (photo)dissociation of it through its early history.”

Of course. The H2O is split into hydrogen and oxygen. The lighter hydrogen escapes. The heaver hydrogen DH20, hydrogen with one neutron or deuterium remains. Oxygen is also lost due to the solar wind stripping or sputtering. Consequently, the H20, DH20 ratio shows how much water has been lost in a planets atmosphere. Venus has much more DH20 than Earth so it has lost a lot more water than Earth since Earth has a magnetic field which blocks the solar wind. The same is true about Mars, a higher Dh20 ratio than Earth’s. Also the temperature on Venus is much hotter due to a much thicker atmosphere of Co2. Plus Earth troposphere traps the water.

My point was that solar wind stripping might not completely remove a exoplanets atmosphere and Venus is a good example since it lacks a magnetic field to block the solar wind. Hopefully, we’ll have some atmospheric spectra to measure with M dwarfs.

Then start branching out. Many papers published since strengthen their points. Most of the old arguments used to rule our M-class suns are rather simplistic–the big one being the idea that a tide-locked world would have its atmosphere collapse on the dark side. Flares can be important, but such activity tends to fade with stellar age. Stellar mass loss is another issue.

“We analyze the evolution of the potentially habitable planet Proxima Centauri b to identify environmental
factors that affect its long-term habitability. We consider physical processes acting on size
scales ranging from the galactic to the stellar system to the planet’s core. We find that there is a
significant probability that Proxima Centauri has had encounters with its companion stars, Alpha
Centauri A and B, that are close enough to destabilize an extended planetary system. If the system
has an additional planet, as suggested by the discovery data, then it may perturb planet b’s eccentricity
and inclination, possibly driving those parameters to non-zero values, even in the presence of strong
tidal damping. We also model the internal evolution of the planet, evaluating the roles of different
radiogenic abundances and tidal heating and find that magnetic field generation is likely for billions of
years. We find that if planet b formed in situ, then it experienced 169 ±13 million years in a runaway
greenhouse as the star contracted during its formation. This early phase could remove up to 5 times as
much water as in the modern Earth’s oceans, possibly producing a large abiotic oxygen atmosphere.
On the other hand, if Proxima Centauri b formed with a substantial hydrogen atmosphere (0.01 –
1% of the planet’s mass), then this envelope could have shielded the water long enough for it to be
retained before being blown off itself. After modeling this wide range of processes we conclude that
water retention during the host star’s pre-main sequence phase is the biggest obstacle for Proxima b’s
habitability. These results are all obtained with a new software package called VPLANET.”

In Centauri Dreams, Paul Gilster looks at peer-reviewed research on deep space exploration, with an eye toward interstellar possibilities. For the last eleven years, this site has coordinated its efforts with the Tau Zero Foundation, and now serves as the Foundation's news forum. In the logo above, the leftmost star is Alpha Centauri, a triple system closer than any other star, and a primary target for early interstellar probes. To its right is Beta Centauri (not a part of the Alpha Centauri system), with Beta, Gamma, Delta and Epsilon Crucis, stars in the Southern Cross, visible at the far right (image: Marco Lorenzi).

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